1. Field of the Invention
This invention relates to an epoxy resin composition for encapsulating a semiconductor and to a semiconductor device therewith.
2. Description of the Related Art
An epoxy resin composition has been predominantly used for encapsulating a semiconductor device because it has well-balanced properties such as productivity, a cost and reliability. In surface mounting for such a semiconductor device, the semiconductor device is suddenly exposed to an elevated temperature of 200° C. or higher during a solder dipping or solder reflow process, leading to explosive vaporization of absorbed moisture. Such vaporization may generate a sufficient stress to cause peeling in an interface between each plated junction on a semiconductor element, lead frame or inner lead and a cured epoxy resin composition, or cracks in a semiconductor device, leading to significantly lower reliability.
For solving the problem of deteriorated reliability due to soldering, there has been commonly used a technique that a content of an inorganic filler in an epoxy resin composition is increased for reducing moisture absorption, improving strength and reducing thermal expansion to improve soldering resistance and a resin with a lower melt viscosity is used to maintain a lower viscosity and higher flowability during molding.
Meanwhile, adhesiveness in an interface between a cured epoxy resin composition and a base material such as a semiconductor element and a lead frame within a semiconductor device has been significantly important for improving reliability after soldering. When adhesive strength in the interface is weak, peeling may occur in the interface between the cured epoxy resin composition and the base material after soldering, leading to cracks in the semiconductor device.
In order to improve soldering resistance, there have been attempts for improving adhesiveness at the interface between a base material and the cured epoxy resin composition by adding a silane coupling agent such as γ-glycidoxypropyltrimethoxysilane and γ-(methacryloxypropyl)trimethoxysilane to the epoxy resin composition. However, these silane coupling agents alone have become inadequate to meet more strict requirement in soldering resistance in association with a higher reflow temperature during mounting and the use of a pre-plating frame made of Ni, Ni—Pd or Ni—Pd—Au in response to a lead-free solder.
For dealing with the problem, there have been suggested surface treatment of a lead frame with an alkoxysilane coupling agent (e.g., see Japanese Laid-open Patent Application No. 1994-350000) and a resin composition and a resin-encapsulated type semiconductor device containing a thiazole, sulfenamide or thiuram compound (e.g., see Japanese Laid-open Patent Application Nos. 1987-209170 and 1987-260344). However, the silane coupling agent described in Japanese Laid-open Patent Application No. 1994-350000 has a drawback that it is unstable at an elevated temperature, leading to insufficiently improved adhesion after soldering resistance treatment. The compounds described in Japanese Laid-open Patent Application Nos. 1987-209170 and 1987-260344 have a higher molecular weight and contain many unstable bonds such as nitrogen-sulfur bond, and thus it is believed to be possibly decomposed in a encapsulating resin after molding.